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Oct 31, 2010

Jetting, Mikuni TMX S-7 Nozzle Swap, 2001 CR250

Although this was done on a 2001 CR250, the same basic principles apply to most carbuerated bikes.
In a previous post, "Carb Jetting Basics" I explained why jetting is important, what factors affect jetting, and how to do basic jetting adjustments. I also stated that in most cases when doing jetting the needle and/or the needle jet does not need to be changed. Although this is true, there are conditions that do require a needle and/or a needle jet change. The diagram to the right shows that through the throttle range, different parts of the carburetor come into play to help achieve the correct mixture from idle to wot(wide open throttle). As you can see, by changing the idle jet, main jet, and by dropping or raising the clip on the needle, it is possible to adjust somewhat the entire range. However, if you find that you are dropping or raising the needle to the last slot and still not getting the results you want it becomes necessary to change the needle and/or the nozzle(needle jet). There are 3 different elements that can be changed to get the desired results. The first 2 have to do with the needle. One is the straight diameter of the needle, and the second is the taper of the needle. The straight diameter has more effect from idle to half throttle while the taper controls from roughly 1/4 to 3/4 throttle. You can even buy needles with changing tapers to further control how fuel is metered, but again, in most cases, you can get good results with a single taper. A bigger needle will lean the mixture, where a thinner one will richen the mixture. Also, lowering the needle(raising the clip) will lean it, and raising the needle(lowering the clip) will richen it. The nozzle is the 3rd element that can be changed to either richen or lean the mixture. In my case, I needed a leaner mixture. If your bike is stock, with the stock s-9 nozzle, and you are riding at higher elevations, chances are good that you'll have the same problem. Since every bike is different, and there are so many variables, it is nearly impossible to say you can jet your bike just like mine and have the same results, but using someone else who has a similar bike and rides in similar conditions can get you very close and at least get a place to start.

The picture to the left is a good illustration of how these parts actually work to control the mixture.

What are some good indications that you might need to worry about the nozzle and/or needle? As I said, first jet the main and idle, then if you are on or close the the lowest position on your needle, but you still are rich in the mid range, then you should consider it. You'll know you are rich if you are still fouling plugs and they are black when you change it out. Also, if your bike tends to run well wide open, but loads up in slower technical stuff where you are in the low to mid throttle, then you'll know your still rich in that range as well. After doing a plug chop at full throttle, I determined my main was right on. The plug was the right color and it had good smooth power. I decided to change the nozzle and the needle The stock nozzle(needle jet) is a S-9. The size is denoted by the letter and the number and dictates the jet's inside diameter. The leanest is the N-0 @ 2.55mm(inside diameter). From there and alphabetical increase in size code indicates an inside diameter .05 larger than the original. A numerical increase of the size code indicates a change of .005mm. In my case, the stock S-9 has a size of 2.845mm, and the S-7 would be 2.835mm (an S-8 would be 2.84mm). The S-7 has been regarded as a good choice and an all around decent fix for the mikuni tmx carb if you ride in the 4,000 or above elevation range for the 2001 CR250. The stock needle for a 2001 CR250 is a 6BEH1-73. The last two numbers in the part number determine the straight section diameter of the jet needle, otherwise known as the original diameter or root diameter. In the case of the stock 2002 CR250 needle, 6BEY30-74 (for example) the straight section diameter is 2.740mm Going from a -74 to a -75 will lean the mixture at approximate 1/8 to about 3/4 throttle (2.740mm to 2.750mm). The 6 at the beginning is the needle series number designating that this needle is used for a TMX 38mm carb (and some others that use the 6 series needles). The letters ‘BEY’ are the taper of the needle. The first letter of the alphabet, 'A' represents 15" (minutes) of taper, with each succeeding letter representing an additional 15" (minutes) of taper. So in this case 'B' would be 30", 'E' would be 2*15", and 'Y' would be 6*15". The other numbers (30) are seemingly placed at random and have not had any type of pattern associated with them in order to decode them, BUT an increase in this number, say for example part no. 6BEY31-74 would be 1/2 clip position leaner than the stock needle. I ordered a 6BEH1-74, or one step leaner for my bike. I have included catalog pages from Sudco International, that can also be found here (pg 114, pg 115). At the time I placed the order, they didn't have needles, so I ordered one from cheap cycle parts, in the OEM section. Keep in mind while doing changes with your jet needle and/or needle jet, that all these elements overlap in determining the final air/fuel ratio. So, you may need to look at your other adjustments, such as your main and idle jet to make sure you are not too lean at idle or wot. Leaning the needle and needle jet will most likely lean the main and idle circuit slightly as well. If you are not already on the lean side, you should be ok, but it's worth checking again. After all my jetting adjustments, this is the setup I ended up with:

Main Jet: 350
Idle Jet: 30
Jet Needle: 6BEH1-74 (clip, 3rd from the top)
Needle Jet(Nozzle): S-7
Air Screw: 1.5 turns out

Elevation approx 4,250 feet.

Modifications to the bike include the following:
mo-better porting job
FMF Gnarly Pipe
Shorty Silencer
V-Force Reeds
UNI air filter


  1. I've gone one step further than crossed eyed - I have gone cross brained!

  2. :D yea, my brain hurts.